1. Field of the Invention
The invention relates to a process for fine purification of 1-butenic streams which have more than 2000 ppmw (ppm by mass) and less than 8% by mass of isobutene.
2. Discussion of the Background
1-Butene, isobutene, 2-butenes and their subsequent products are obtained in large amounts from technical C4 cuts, for example the C4 cut from steamcrackers or FCC units. These mixtures comprise essentially butadiene, the monoolefins isobutene, 1-butene and the two 2-butenes, and the saturated hydrocarbons isobutane and n-butane. Owing to the low boiling point differences of the ingredients and their low separating factors, distillative workup is difficult and uneconomic. Linear butenes and other products are therefore usually obtained by a combination of chemical reactions and physical separating operations.
The first step, which all workup variants have in common, is the removal of the majority of the butadiene. When there is a viable market for butadiene or it is used in house, it is typically removed by extraction or extractive distillation. Otherwise, it is hydrogenated selectively to linear butenes down to a residual concentration of approximately 2000 ppm by mass. What remains in both cases is a hydrocarbon mixture (so-called raffinate I or selectively hydrogenated crack-C4) which, in addition to the saturated hydrocarbons n-butane and isobutane, comprises the olefins isobutene, 1-butene and 2-butenes (cis and trans).
For the recovery of 1-butene as the target product, the procedure is typically as follows: isobutene is removed very substantially by chemical reaction from raffinate I or hydrogenated crack-C4. The substantial removal of the isobutene leaves a hydrocarbon mixture (raffinate II) which contains the linear butenes and the saturated hydrocarbons isobutane and n-butane, and can be separated further by distillation. In order to be able to use the 1-butene in ethylene polymerization, in which isobutene impurities are undesired, on-spec 1-butene should have a content of isobutene of less than 2000 ppm based on the 1-butene.
A widespread means of chemically converting isobutene is its reaction with alcohols, for example methanol or ethanol, to give the corresponding tertiary butyl ethers. The advantage of this reaction is that the isobutene can be converted virtually fully with high selectivity in the presence of linear butenes. For this purpose, various process technology variants have been developed for the reaction with methanol to give MTBE. The use of MTBE as an octane number improver in gasoline fuels is being regarded increasingly critically. There is therefore an interest in developing methods for the removal of isobutene from hydrocarbon streams which avoids the occurrence of MTBE.
EP 0 048 893 details a process for simultaneously preparing isobutene oligomers and alkyl tert-butyl ether (ATBE) from C4 cuts in a reactor. The catalyst used is an acidic ion exchange resin which is covered partly with metals of the seventh and eighth transition group of the Periodic Table of the Elements in elemental form (oxidation state 0). The products and the unconverted C4 hydrocarbons are separated by distillation. In this process, approximately 8% of the linear butenes are lost by oligomerization. The loss of 1-butene is 7%. However, the main disadvantage of this process is that full isobutene conversion is not achieved, so that the isobutene content in the C4 hydrocarbon fraction removed is too high to be able to obtain on-spec 1-butene therefrom.
DE 25 21 964 describes a two-stage process for preparing alkyl tert-butyl ethers (ATBE) in which, in a first stage, isobutene is reacted with alcohol, the ether formed is removed and the remaining residue is conducted into a second reaction stage.
U.S. Pat. No. 6,472,568 describes a two-stage ETBE synthesis in which at least one stage is a reactive distillation column and in which a molar ratio of ethanol to isobutene of 1.05 is present in the feed to the first reaction stage and a molar ratio of 1.4 to 4.0 in the feed to the second stage.
RU 2167143 describes a two-stage process for preparing ETBE in which a molar ethanol to isobutene ratio of 0.86:1 is established at the inlet to the first stage and a molar ratio of ethanol to isobutene of 5:1 at the inlet to the first reactor of the second stage. The hydrocarbon stream fed to the process has 45% isobutene and 55% other hydrocarbons. What is obtained is ETBE and a hydrocarbon stream which has 1.4% isobutene and which thus, if it were 1-butene, would not satisfy the abovementioned specification.
RU 2168490 describes a two-stage process for preparing ETBE, in which a molar ethanol to isobutene ratio of at least 0.8:1 is established at the inlet to the first stage, and a molar ratio of ethanol to isobutene of from 1.5:1 to 2:1 at the inlet to the second stage. The second stage is designed as a reactive distillation. The stream fed to the process has an isobutene content of 10%. The resulting hydrocarbon stream has 2.1% isobutene and would thus, if it were 1-butene, not fulfill the abovementioned specifications.
In most known processes, after the first stage, the ether obtained is first removed and the remaining hydrocarbon stream is transferred into a second stage. This two-stage procedure is relatively complicated. In addition, the processes have the disadvantage that the 1-butene obtained has a content of isobutene of significantly more than 2000 ppmw.
Starting from this prior art, the object of the present invention consists in the provision of a simple process for preparing fractions comprising 1-butene or 1-butene having less than 2000 ppmw of isobutene (based on the 1-butene) starting from 1-butenic starting mixtures which already have a relatively low content of isobutene of less than 8% by mass.